As is well known, in wide band direct sequence CDMA systems signals consist of different pseudo-random binary sequences that modulate a carrier. Thereby, the spectrum of the signals is spread over a wide frequency range common to a number of channels in the system. Due to the direct sequence coding, orthogonality between signals is achieved, enabling individual decoding of signals from the common frequency range.
This coding principle has many advantages. For instance, direct sequence spread spectrum coding provides substantial reductions of the severity of multi-path fading, which leads to an effective utilization of spectrum resources.
Since signals occupy the same space in the frequency/time domain, power regulation of the individual channels is an important aspect of CDMA systems.
CDMA systems employ power control on both the up- and the downlink. One objective of the power control is to regulate each mobile station transmitter operating within the cell site base-station receiver, such that the signals have the same power level at the base-station receiver regardless of the position or propagation loss of the respective mobile stations. It should be noted that the power level is proportional to the transmission data rate.
When all mobile station transmitters within a cell site are so controlled, then the total signal power at the base-station receiver is equal to the nominal received power times the number of mobile stations.
Each selected signal received at the base-station is converted into a signal that carries the narrowband digital information, whereas the other signals that are not selected constitute a wide band noise signal. However, the bandwidth reduction, which is performed according to the decoding process, increases the signal-to-noise ratio from a negative value to a level that allows operation with an acceptable bit error rate.
The overall system capacity, for instance the number of users that can operate within the cell simultaneously, depends on the minimum signal-to-noise ratio, which produces the given acceptable bit error rate.
On the downlink, the cell also supports power regulation by adjusting the downlink power for each signal to the respective mobiles in response to their requests. The purpose is to reduce power for units that are either stationary, relatively close to the cell site, impacted little by multi-path fading and shadowing effects, or experiencing little other cell interference. Thereby, the overall noise level diminishes and those mobiles being in a more difficult environment will benefit.
The current 3rd Generation Partnership Project (3GPP) specification for third generation mobile telephony system, also denoted UMTS, supports different user data rates for different users. The transmitted downlink power for a given user entity is determined by the interference level in the actual cell, the prevalent channel quality, the user data rate, and the requested quality of service for the data transmission.
In UMTS systems there are two basic kinds of physical channels that are used for transmission: Dedicated channels and common channels. Only one user at a time may use a dedicated channel, whereas many users can share a common channel.
Normally, the downlink interference for a particular user entity stems from a plurality of transmissions of relatively low power to other user entities (on other orthogonal channels). The interference originates from adjacent base stations as well as the very base station or base stations, in case of soft handover, from which the user entity in question receives a transmission.
A transmit power control (TPC) loop is used for dedicated channels. The purpose of the TPC loop is to regulate the downlink power for individual user entities so that the ratio between received power and the interference is held constant even though the absolute value of the interference fluctuates. Thereby, a desired user downlink data transmission quality can be obtained.
The TPC loop makes use of TPC commands that are forwarded from the user entity to the base station once per slot (1 slot corresponds to 0.67 ms). The TPC command may either ‘raise the power’ or ‘lower the power’ in steps. The power step adjustment for each command is normally 1 dB. This means that the TPC loop can adjust the transmission power 1 dB per slot at the most. Thereby, the power transmitted from the base station will vary over time due to variations in interference level from other sources and variations in channel quality. The utilization of the TPC loop will result in a relatively constant interference level for the individual user entity in question.
Recently, a new downlink service, High Speed Downlink Packet Access (HSDPA), has been introduced in 3GPP. A brief account of the operating principle can be found in “Performance Aspects of WCDMA Systems with High Speed Downlink Packet Access HSPDA)”, by T. E. Kolding, et. al.
The HSPDA transmission makes use of a 2 ms transmission time interval (three time slots), adaptive modulation and coding (AMC) multimode transmissions, such as QPSK and 16 QAM modulations, fast physical layer (L1), hybrid automatic request (H-ARQ). The scheduler is transferred from the radio network controller to the so-called Node B, also denoted base station set, BSS. In FIG. 6, an outline has been given, indicating the data transmissions, up-link power control and downlink power control to various user entities, UE's.
FIG. 1 shows the major channels utilized in HSPDA.
On the downlink side there is provided: Several common data channels 1. A dedicated signal radio bearer 2 for each user using HSPDA transmissions; a common channel for control signaling 3, a number of common user data channels 4-5, which are allocated HSPDA data in a flexible manner.
On the uplink side there is provided: a dedicated channel for, among other things, providing channel quality information, CQI, and HSPDA automatic request signaling, H-ARQ, 6, an uplink dedicated channel associated with each HSPDA user comprising both control information and data, 7.
With the introduction of High Speed Downlink Packet Access (HSDPA) in UMTS systems, the interference level will not longer fluctuate in a slowly manner. Large momentary interference steps of several dB's will appear when the HSDPA channel turns from no data transmission to maximum data rate transmission. Other mobile stations will experience performance degradations around the time of initiation of the high power HSPDA transmission. This problem is often described as the ‘flashlight effect problem’.
In FIG. 2, an exemplary scenario for the downlink interference level has been depicted for a typical user entity. The user entity experiences a certain level of thermal noise, N_TH. Also interference from downlink channels of adjacent cells, I_ADJ. Moreover, non-HSPDA inference from other downlink channels in the cell in which the given user entity resides, I_NON_HSPDA_CELL, also contributes to the interference level. The latter level is often of a considerable level, in relation to the two first mentioned sources. Finally, the interference from non-regulated HSPDA transmissions is shown, I_HSPDA_CELL. As mentioned above, these transmissions may be of a high magnitude and may change abruptly.
In FIG. 3, the HSPDA transmission of FIG. 1 corresponding to the used HSPDA power in Node B has been shown, further.
In FIG. 4, the sum, D_PWR, of the interference contributions of FIG. 2 has been depicted for an unregulated HSPDA transmission. The given actual dedicated channel power is denoted A_PWR. Since the TCP caters for a maximum change of 1 dB/0.67 ms, the prevalent signal to interference level, S/I_1, may decrease below the given minimum detection threshold on rising flanks of the HSDPA generated interference.